The present invention relates to silicone-derived solvent stable membranes. Microfiltration (MF), Reverse Osmosis (RO) and Ultrafiltration (UF) membranes are often made from polymers, which swell and dissolve in organic solvents. Such polymeric membranes may dissolve in different solvents, but generally the best solvents for dissolving the polymers are dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethylsulfoxide (DMSO), hexamethyl phosphoramide, sulfolane (tetramethylene sulfone) and N,N-dimethylacetamide.
There are many potential membrane applications which could usefully employ solvent stable membranes. Such applications are e.g. in the areas of food technology, biotechnology, the treatment of waste streams, chemical manufacturing and petrochemicals. These solvent stable membranes are desirably also swelling resistant, because swelling of such membranes under pressure would indicate solvent/membrane interaction and thus polymer chain mobility, which usually results in compaction, and loss of flux and of performance generally, under pressure. A particular advantage of such solvent stable membranes would be, that they could be exposed to a variety of solvent media including aqueous solutions, suspensions or emulsions, as well as to organic solvents which contain solutes.
There are presently available solvent stable membranes in the form of ceramics or other inorganic materials and specialized crosslinked polymers. These, however, are expensive, difficult to prepare, brittle, cannot be made conveniently with the desired molecular weight cutoffs, and in practice are restricted to certain configurations. Besides ceramics, there are available membranes from crosslinked polymers such as epoxy polyimide type polymers, as well as and encapsulated polymers. Encapsulated polymers are described in U.S. Pat. No. 4,778,596; the membranes are coated on the external surfaces and on the internal pore surfaces with an aqueous polymer which is then crosslinked. The support membrane is not crosslinked, but encapsulated in an outer skin; such membranes swell but do not dissolve, upon immersion in organic solvents. Crosslinked polyimide commercialized by NITTO (Japan) have some solvent resistance but limited pH/temperature stability; in addition, the membranes are limited to the UF range with low efficiency in operation.
Other organic polymers (polysulfones, polyamides and cellulose acetate) may be cast into similar membranes, of different configurations, but they are difficult to insolubilize, and may swell after insolubilization. Many new organic polymers, in addition to the above-mentioned polyimides e.g. (polyetherketones, polytriazoles, perfluoropolymers such as perflurodioxolanes) may be cast from esoteric or difficult-to-handle solvents and demonstrate insolubility in many solvents, but they still swell in many of these solvents, and even dissolve in some others. Even after cross-linking, the degree of swelling may not be reduced below 10%.
Silicone composites are known. Thus, U.S. Pat. No. 4,243,701 (Riley) discloses thin films of dimethylsilicone on various supports, particularly polysulfones. These membranes are designed mostly for gas separation. Other gas separation membranes using silicone layers are described in U.S. Pat. No. 4,230,463 (Henis et al, use of polydimethylsiloxane); Ward et al, J. Membr. Sci., 1976, 1: 99-108 (use of ultra thin silicone polycarbonate); U.S. Pat. No. 4,950,314 (Shinichi et al, use of polyolefins and polyarylene oxides containing active silyl groups which crosslink the thin coating through oligosiloxane bonds of 1-3 units); EP 0214792A (Cabasso et al, use of derivatives of aminopolysiloxanes with brominated derivatives of polyphenylene oxide); EP 0041839A (Sugie et al, use of silicone-containing copolymers); EP 0099432A (Hirose, use of silarylene-siloxane polymers). Also, the use of a methyl terminated polysiloxane as drying agent for cellulose acetate has been described in U.S. Pat. No. 4,855,048. The entire disclosures of all these patents and publications are explicitly incorporated herein by reference. While in the context of the present invention, the issue of solvent stable substrates is of primary importance, this is not the case in the context of the mentioned references.
Baker et al, in U.S. Pat. No. 4,553,983 and J. Membr. Sci., 1987, 31: 259-271, describe thin polydimethylsiloxane films of 0.1 to 10 micron thickness, on polyimide and polysulfone supports. These supports are described as solvent resistant, and the polyimide membrane is described as crosslinked polyimide. In reality, these membranes swell extensively in many solvents and from a practical viewpoint could not be regarded in general, as solvent stable composites. Other membranes useful for supports according to Baker are to be found in Strathmann et al, in Desalination, 1975, 16: 179; these are solvent resistant and will not dissolve in many solvents, but they will swell beyond 10% in many of the same solvents. Kimmerle et al, in J. Membr. Sci., 1988, 36: 477-488, describe polysulfone hollow fibers coated internally with a thin layer of dimethylpolysiloxane. These, and the membranes described by Baker are designed for removal of organic solvents from air, gas, and aqueous waste streams. For aqueous/solvent streams, the process is "pervaporation". The silicone layers of all of these membranes are cured and crosslinked by well-known techniques. The disclosures of these patents and publications are incorporated by reference herein. Solvent stable acrylonitrile-derived substrates are described in allowed U.S. Ser. No. 07/415,156, the entire contents of which are incorporated herein by reference.
The silicones useful in the present invention are optionally silanol-terminated polymers and prepolymers, including aliphatic and aromatic polysiloxanes, both mono- and di-substituted, containing aliphatic or alicyclic groups such as lower alkyl (C.sub.1 -C.sub.6), e.g. methyl, ethyl and propyl, cycloalkyl (C.sub.3 -C.sub.8), lower alkoxy (C.sub.1 -C.sub.6), C.sub.6 -C.sub.12 carbocyclic aryl or aryloxy such as phenyl, naphthyl, phenoxy or naphthoxy, or C.sub.1 -C.sub.6 alkanoyl or alkanoyloxy such as acetyl or acetoxy, or C.sub.7 -C.sub.13 carbocyclic acryl or acyloxy such as benzoyl, naphthoyl, benzoyloxy or naphthoyloxy. However, for the pore-protecting intermediate layer, silanol-terminated silicones are utilized.
The silicones, when used for the outer layer of the composites of the invention, may be crosslinked in the presence of a crosslinking agent and/or a crosslinkable comonomer such as methylstyrene. Typical crosslinking agents when olefinic bonds are present in the silicones are organic and inorganic peroxides. On the other hand, silanol-containing or -terminated siloxane polymers can be crosslinked with an alkoxysilane such as a tetraalkoxysilane, trialkoxysilane or polyalkoxysiloxane. Silanol-terminated polymers may also be cured by polysiloxanes containing silanic hydrogen. By a suitable choice of catalyst, cure may be effected at room temperature in times ranging from 1.0 minutes to 42 hours. Presently preferred catalysts are stannous octoate, and dibutyltin dilaurate. Other catalysts are dibutyltin dioctanoate, dibutyltin diacetate, salts of carboxylic acids such as iron 2-ethylhexanoate and cobalt naphthenate, titanic acid esters, and amines such as ethylamine, dibutylamine and pyridine.
A presently preferred silicone for use as the outer layer of the present composites is a crosslinked polydimethylsiloxane, which may for example be prepared from a silanol-terminated prepolymer and a crosslinking agent which may be e.g. a silane or a siloxane crosslinking agent having four or more functional groups. A single coating solution can contain silanol-terminated polysiloxane, crosslinking agent and catalyst. These and other methods are described in the above mentioned U.S. Pat. No. 4,950,314. One preferred combination is silanol-terminated dimethylsiloxane (MW 36,000) with tetraethoxysilane and dibutyltin dilaurate, coated onto a substrate (previously treated with pore protector) from an aliphatic hydrocarbon solvent such as hexane, or from perfluoro solvents.
Commercially available polysiloxanes have molecular weights between 1000 and 300,000, although the invention is not limited to this range. The silicone polymer may be applied to the pore-protected support in many different ways known in the art of coating thin films onto porous supports. Such methods are described in previously mentioned U.S. Pat. Nos. 4,243,701, 4,230,463 and 4,950,314 and in J. Membr. Sci., 1976, 1: 99. One presently preferred and relatively simple method is dipping the pore-protected substrate into a solution of silicone polymer or prepolymer, draining and curing. The concentration of silicone in such solution may vary from 0.01 to 10%, but is preferably in the range of 0.1 to 2%, for both the pore-protecting step and the final coating step. The concentration of the crosslinker may vary between 0.05 and 10%, preferably 0.1 and 5%. The solvents for the final silicone coating may be e.g. aliphatic hydrocarbons or perfluoro solvents and for the pore protector, e.g., lower (e.g. C.sub.1 to C.sub.4) alcohols; or the same solvent could be used for both the pore protecting step and for the final coating step.
The composite membranes of the invention find application, e.g., for the concentration and purification of organic solutes, such as dyes, dye intermediates, optical brighteners, antibiotics, peptides, proteins, enzymes, hormones, & herbicides, in organic solvents or in aqueous/organic mixtures.
Moreover, liquid streams which can be treated by means of solvent stable membranes of the invention include also:
(i) Lubricating oils, which are in particular low MW components having a MW cut off in the range of 300-2000 Daltons, and which are dissolved in strong organic solvents such as NMP, phenol, MEK, MIBK, toluene and their mixtures. Their separation requires availability of solvent stable membranes which will be stable in solvents such as those specified, and which will retain the dissolved low MW oils to a sufficient degree, e.g. between 70-95%. Thus, merely by way of example only, a solution of 10-15% paraffin oil dissolved in a 1:1 MEK/MIBK mixture may be separated into a stream containing a twofold concentration of oil and a stream containing only 10% of the original concentration.
(ii) Catalysts dissolved in organic solvents. Several catalysts comprising metal organic complexes are in commercial use for performing catalytically enhanced polymerization reactions in organic media. These catalysts are very expensive and there is great interest in recovering them from reaction mixtures. The molecular weight of the catalyst may vary from 200-300 and up to 2000-3000 Daltons.
(iii) Low MW oligomers in paint wastes dissolved in strong organic solvents such as MEK, butyl acetate and/or other strong solvents singly or in admixture.